Tag: comet

So there’s this comet named 168P/Hergenrother. It’s one of a bazillion such iceballs orbiting the Sun, but this one turns out to be more interesting than most. For one thing, it has a short period, orbiting the Sun once every 6.8 years or so. Its orbit goes out to about that of Jupiter’s, and reaches down into the inner solar system about as far as Mars. It never gets closer than about 80 million kilometers (50 million miles) to us, so it’s usually relatively faint, and you need a big ‘scope to observe it.

It was discovered in 1998, and made a second pass down our way in 2005. This year, 2012, it came by again, and folks around the world observed it as they do any comet. But then, in September, it gave us a surprise. A big one. Lots of observers were reporting that practically overnight the comet grew hugely in brightness, getting as much as 700 times brighter than expected! Not only that, but observations showed the shape of the comet had changed, going from fairly point-like to much fuzzier.

That could mean only one thing. The comet was breaking up.

The picture above is from the Faulkes Telescope North, located on the Hawaiian observatory on Haleakala. It’s a composite of lots of separate exposures that were added together; you can see the stars are trailed (actually stippled; each exposure was short but then shifted to line up on the comet). The comet is the bright fuzzy blob in the upper right, and if you look just below the main part you can see a second fuzzy blob, much fainter.

First, don’t panic. We’ve seen this happen to comets before, and this one is so far away from us we’re in no danger at all. It literally cannot get near us.

Second, it’s very interesting scientifically. Comets are basically big frozen snowballs peppered with rock. Imagine scooping up a handful of gravel and snow and then packing into a loose ball. That’s a comet, if your snowball is several kilometers across and the ice is actually frozen water and carbon dioxide. When they are far from the Sun comets stay frozen and are exceedingly dim. When they get closer, the ice goes directly to a gas (called sublimation), and escapes from the solid part (called the nucleus). It expands and can form a big fuzzy head around the solid nucleus that can be tens of thousands of kilometers across, bigger than planets! This is also what gets blown back by the solar wind (and the pressure of sunlight) to form the tail(s) of the comet.

This means that every time a comet gets closer to the Sun and starts to sublimate, it dies a little bit. Material leaves the comet and never comes back. But that ice is what holds the comet together! So sometimes enough ice turns into a gas and escapes that the comet gets substantially weaker, and big chunks of it can dislodge, falling away. That’s what appears to have happened to Hergenrother.

While we’ve seen this before with other comets, it’s not like it happens every day, so any chance to see this occur is fascinating. In 2006 we watched as comet 73P/Schwassmann-Wachmann 3 literally disintegrated. Even more amazing, in 2007 the run-of-the-mill comet 17P/Holmes suddenly erupted, getting hugely brighter, and a huge shell of dust was seen to be expanding around it. Now we think Holmes collided with a small asteroid, and the violence of the event blasted off the material. I saw Holmes with own eyes when this happened, and even though it was past the orbit of Mars, the shell of dust was easily visible to the naked eye. It was awesome.

No two comets are ever really alike. They have different sizes, shapes, compositions, and orbits. And each will behave slightly differently as they round the Sun and head back into deep space. If there’s a lesson from Hergenrother, it’s this: it’s always a good idea to keep an eye on everything in the sky. Just because something looks routine now doesn’t mean it won’t try to pull a fast one later.

You spend most of the time — billions of years, really — out in deep space where it’s cold and dark. Of course, since you’re mostly made of ice, that’s not so bad. After all, the Sun is hot, and if you venture too close…

Well, you know what happens then. And such was the fate of Comet SWAN, discovered just a few days ago as it plunged headlong into the seething fires of the Sun. And I have video!

That was made from images taken by NASA’s SOHO satellite. In fact, the comet is named SWAN because it was first seen in the SOHO SWAN camera, designed to look for ultraviolet light coming from hydrogen. Here’s the thing: no comet has ever been seen before in that camera, including the phenomenally bright comet Lovejoy from a few months ago. But Lovejoy got incredibly bright overall, while this new comet never did brighten much. Comet SWAN must have undergone some sort of outburst to make it so bright and then fade again; that’s happened before.

Here’s another shot of it from SOHO:

[Click to enhalleyenate.]

Comets like these are called Kreutz family Sun grazers, a collective group of comets on similar orbits that take them very close to the Sun’s surface. Some survive, like Lovejoy did, and some… don’t.

Isn’t that lovely? The comet itself is a bit smeared out since it moved over the time as the picture was taken. But even so, wait a sec — you may have noticed something else odd about this picture. Comets have a tail, right? So why do you see two tails, a blue one pointing off to the left and the other reddish, pointing off to the right?

Aha! Oh, I love a chance to lecture a bit. Bear with me. This is cool.

As I said, comets have a lot of ice in them. As they near the Sun that ice warms, and turns directly into a gas (that process is called sublimation). This gas expands away from the solid nucleus, forming a fuzzy cloud called the coma (Latin for "hair").

Now this is where things get interesting. This coma has both gas in it as well as dust and grains of rock carried off as the ice goes away. The Sun blows out a wind of subatomic particles called the solar wind. This ionizes the gas — strips off one or more electrons — and that gas then gets dragged along with the solar wind. That wind is moving, traveling at several hundred kilometers per second, far faster than the comet moves. So that tail gets blown directly away from the Sun. It tends to be blue (or sometimes green), due to the ionized gas in it.

But the dust and rock isn’t affected as much. As it moves off the comet, it tends to lag behind a bit, following the comet in its orbit. This material reflects sunlight and also reddens it a bit, so that makes the dust tail look yellow or red.

The Sun is in the center, and the planets are labeled; I deleted the orbits for all the planets except Earth and Jupiter so you can get a sense of the plane of the solar system. The comet is in blue, and as you can see its orbit is not at all aligned with the planets; it punches upward through the plane on the right, and then plunges back down on the left. It may be hard to get a 3D image of this in your head, but I added in the two tails: the blue ion tail pointing away from the Sun, and the redder dust tail lagging behind the comet itself. From the viewpoint of the Earth, "underneath" the comet, the tails appear to be on opposite sides of the comet and pointing in opposite directions! It’s just perspective making it look that way; at this point in the comet’s orbit the tails are actually closer to 90° apart.

Strange, isn’t it? I’ve found that three-dimensional thinking is one of the tougher barriers to people really understanding how objects move in space (that, and the vast physical scale of space that crushes our minds to dust). But perspective counts! In astronomy, as well as life itself. And when you get a little perspective, why, sometimes things are even cooler than you first thought.

See it? It’s faint, but there. Actually, there are a lot of observations from multiple observatories and detectors, which allowed astronomers to find out quite a bit about this doomed chunk of ice and rock.

For one thing, it was screaming along at about 650 kilometers per second (400 miles/second) as it flamed out. To give you an idea of how flippin’ fast that is, it would’ve crossed the entire United States in about eight seconds.

Yeah, I know.

It also passed an incredible 100,000 km (62,000 miles) above the Sun’s surface. Have you ever stood outside on a hot day, and thought the Sun would cook you? Now imagine the Sun filling half the sky. That’s what that comet saw. No wonder it disintegrated.

As it approached the Sun, it was watched by NASA’s Solar Dynamics Observatory. In its final 20 minutes or so, the comet broke up into a dozen pieces ranging from 10 – 50 meters in size (and no doubt countless smaller ones too small to detect), with a tail of vaporized material streaming behind it that went for thousands of kilometers. For that size, it would’ve had a mass of hundreds of thousands of tons — about what a loaded oil tanker weighs on Earth!

We’ve learned a lot about how comets break up and disintegrate by observing this event, but it’s raised further questions: like, why did we see this at all? Comets are faint, and to be able to see it this way against the bright Sun is odd. It was definitely one of the brightest comets seen, but it’s interesting to me that it appears to glow in the ultraviolet, as it did in the above video. That means, at that wavelength, it was brighter than the Sun! It wasn’t like a meteor, burning up as it slammed through material, so some other process must have affected it. I suspect that the Sun’s strong magnetic field may have had something to do with it; in the far ultraviolet magnetism is a strong player. Gas under the influence of intense magnetic fields can store a lot of energy, which is why sunspots — themselves the product of magnetic squeezing — look bright in UV.

Perhaps as the comet broke up, the particles inside got excited by the magnetic fields of the Sun and glowed. I’m no expert, and I’m spitballing here. The thing is, no one is exactly sure. But that doesn’t mean we won’t find out. Nothing makes a scientist’s noggin itch as much as a mystery like this, something apparently misbehaving.

One of the single most important words in science is "yet". We don’t know yet. But we will. Someone’ll figure this out, and we’ll have one more victory in our quest to better understand the Universe.

Back in the day, it used to be that most new comets and asteroids were discovered by astronomers diligently sitting at their eyepieces, spending one cold night after another patiently scanning the skies. The advent of robotic astronomy changed that, and now the vast majority of all celestial newcomers are found automatically.

But Australian "amateur" astronomer Terry Lovejoy changed that last week: not only did he discover a comet — which isn’t that unusual, though still cool — but it turns out to be a sungrazer, a comet that plunges deep down to the center of the solar system, practically skimming the Sun’s surface.

Here is Lovejoy’s discovery image:

This is a combination of three images; the comet moves between exposures a bit so he re-centered the comet in each shot and added them together. It’s the fuzzy blob in the middle of the frame. The comet’s official name is C/2011 W3 (Lovejoy), and on December 16th it will pass just about 880,000 km (500,000 miles) from the Sun’s surface — only a little bit more than than the radius of the Sun itself! 180,000 km (110,000 miles) — less than half the distance from the Earth to the Moon!* This may be a death dive, since many such comets don’t survive the intense heat of the Sun from that distance. Comets are composed of lots of rock held together by ice, so when the ice vaporizes, the comets disintegrates.

If you’re a fan of over-the-top ridiculously huge violent explosions, then you won’t do any better than gamma-ray bursts. With apologies to Douglas Adams and Eccentrica Gallumbits, GRBs are the Universe’s largest bangs since The Big One. When they were first discovered, during the Cold War, it was unclear what caused them. There were more theories than there were observations of them! Now we’ve observed hundreds of these things, and we’ve learned quite a bit about them, like a) every one of them is different, 2) they have lots of different sources, and γ) even after five decades they can still surprise us.

Last year on Christmas, the light from a gamma-ray burst reached Earth and was detected by NASA’s orbiting Swift satellite. Designated GRB 101225A, it was weird right off the bat: it lasted a staggering half hour, when most GRBs are over within seconds, or a few minutes at most. Followup observations came pouring in from telescopes on and above the Earth, and the next weird thing was found: the fading glow from the burst seemed to be coming from good old-fashioned heat: some type of material heated to unbelievable temperatures. Usually, the afterglow is dominated by other forces like rapidly moving super-intense magnetic fields that accelerate gigatons of subatomic particles to huge speeds, but in this case it looked like a regular-old explosion.

Both of these things are pretty dang weird. So what could have caused this burst?

Normally, we think GRBs are the birth cries of black holes. When a giant star explodes, or two tiny but ultra-dense neutrons stars merge, they can form a black hole and send vast amounts of gamma rays (super high-energy light) sleeting out into the Universe. In this case, though, something different happened, and two ideas of what was behind it are emerging…. but both involve neutron stars. And I’m not sure which idea is cooler.

A few days ago, three astronomers from Mexico posted a paper online (PDF) claiming that an observation from 1883 indicates a small comet passed within a few thousand kilometers of the Earth’s surface, and perhaps as close as 500km! Had this hit us, we would’ve been hammered by thousands of explosions as powerful as the largest nuclear explosions ever detonated.

The thing is, I’m not buying it. While superficially the interpretation fits the observations, there are way too many problems with it.

Here’s the deal. During the days of August 12 – 13, 1883, a Mexican astronomer named Jose A. y Bonilla reported seeing hundreds of objects passing directly in front of the Sun. They were small, appeared fuzzy, and left behind a misty appearance. In total, Bonilla says he saw 447 such objects!

The authors of this new work claim that what Bonilla may have seen was the remnants of a small comet that had previously fragmented. We’ve seen comets do this, and in fact it’s somewhat common. In 2006, Hubble took the picture shown above of the comet 73P/Schwassmann-Wachmann 3, which had recently disintegrated. So that part isn’t too far-fetched. However, once you make that assumption, things get pretty dicey.

The authors use the observations by Bonilla to estimate the distance and size of the comet fragments. Bonilla observed these objects at an observatory in Zacatecas, Mexico, but they were not seen transiting the Sun by any other observatories anywhere else. This can be used to narrow down their location; it means they must have been close to Earth. Had they been far away then other observatories would have seen them moving across the Sun. It’s like a bird flying by just outside your window; someone looking out a different window wouldn’t have seen it, but a bird a few hundred meters away would be visible to both.

Doing some simple math, the authors calculate the comet fragments were no closer than about 500 km (300 miles) from the Earth’s surface, and no farther than about 65,000 km (40,000 miles).

This right there is enough for me to be extremely skeptical of this idea. When a comet breaks up, it spreads out. Even when intact, the material surrounding a comet can be tens or even hundreds of thousands of kilometers across! Claiming that a comet broke apart, yet managed to constrain its pieces to volume of space less than a few thousand kilometers across strains credulity.

On October 1, a bright comet screamed into the Sun, and apparently disintegrated. This happens pretty often, actually, but in this case, just minutes later, the Sun blew out a pretty hefty coronal mass ejection, a huge explosion of magnetic energy that can release billions of tons of material.

Some people have speculated that these two things are related (including times when this has happened in the past). Are they? We have videos of the event from three different satellites, giving us three angles on what happened, providing clues on what really occurred.

To shed some light on this — haha — I made a short video explaining this, including the footage of the comet collision and CME as seen by the three satellites:

[It helps to set the video resolution to 720p to see the details in the satellite views.]

So my guess is that while it’s possible, it’s not probable. CMEs happen all the time, so I’d expect a few to happen around the same time as comets flying past the Sun just by coincidence. We don’t have any physical reason to think they’re related, and when they are examined more closely, the CMEs usually don’t come from a spot near the Sun where the comet traveled. Still, it’s worth looking into, at least to build up a statistical case on way or the other.

A pair of astronomers monitoring an all-sky camera got a surprise (PDF) when they checked data from last February: a half dozen meteors all seemed to come from the same spot in the sky, indicating they all had a common origin. After doing some calculations, they found that they probably come from a parent comet with an orbit that’s at least 53 years long. Moreover, the orbit of this comet crosses that of the Earth, meaning we may have a close encounter with this object sometime in the future.

And because I can sense the oncoming panic on the web over this news, let me break it down for you. I’ll give you the science (which is cool), how we know this unseen comet may be potentially, um, interesting, then the reason you don’t need to run around in circles screaming (spoiler: it’s rude to others nearby, but also unnecessary).

But just to be up front: should you panic? Nope. We know there are objects out there that could hit us in the future sometime. This comet is in many ways just another one. As I’ll point out below, we pass through lots of meteor streams, so there are plenty of other comets that could hit us. I know, I know, that doesn’t sound reassuring, but think about it: how often is the Earth hit by a comet? Not very often, despite having a few on the list of Potentially Hazardous Objects. So having one more we know about out there isn’t great, but in reality doesn’t really make things any worse for us.

Meatier showers

That picture above is one of the meteors in question. You can see the streak as the tiny bit of rock (probably the size of a grain of sand) glowing as it rammed through the Earth’s atmosphere at about 35 km/sec (22 miles/sec, or nearly 80,000 mph). If you go out on any dark night, you’re bound to see the random meteor or five. But meteor showers are when we see lots of them in a short time, and they occur when the Earth passes through the dust debris left behind by a comet.

Most comets are dirty snowballs: dust, pebbles, and boulders held together by ice (water ice, but also frozen carbon dioxide and other things we normally think of as gases). This makes comets the litter bugs of the solar system, shedding material when the Sun warms them up and turns the ice into gas. The vapor blows off, and the looser material forms a ribbon or stream that stays more or less along the same orbit as the comet.

If the path of the comet intersects the orbit of the Earth, we plow through that material at the same time every year. Read More

It’s not known how many comets orbit the Sun in our solar system, but the number may be in the trillions. They spend a long, long time in the deep reaches of the outer solar system, only occasionally plunging toward us. If they pass near a planet their orbit can be changed, and some wind up on paths that take them so close to the Sun they burn up. These are called sungrazers.

That is what NASA’s Solar Dynamic Observatory saw on the evening of July 5/6, 2011. This has been seen many times before, but this is the first time one has been seen streaking directly across the Sun’s face!

Here’s the video (I recommend watching it in HD — at least 720p –to make the comet easier to spot):

Did you see it? The whole event took about 20 minutes to unfold, and is seen here highly compressed in time. This is no perspective effect; that comet really was just above the Sun’s surface, and most likely impacted the Sun or disintegrated from the heat. Astronomers are even now going over the data from the event to see if they can determine the comet’s fate.

On the NASA Sun-Earth news site is more information, and a very cool video from SOHO showing the comet’s approach to the Sun.